1. Field of the Invention
The present invention relates to a device for reciprocating an object by a driving means such as wire or the like, and particularly where movement at a constant speed is required. The invention is more specifically directed to an improvement in a reciprocating driving device having a shock absorbing feature for absorbing wire vibration caused by the initial shock at the start of motive action, and is of particular use in a copying machine such as an electrophotographic reproducing apparatus.
2. Description of the Prior Art
In a copying machine, a copy board (in a moving copy board type machine) or an optical member such as mirrors (in a moving optical system type machine) is driven by the revolving torque of a motor or direct coupled clutch through a power transitting means of wire or the like. This arrangement, however, results in one relatively tight and one relative loose wire portion being produced on respectively opposite sides of the driving pulley.
Furthermore, when the driving direction is reversed, the loose side of the wire suddenly becomes tight, while the tight side is correspondingly loosened. The inertia of the driven object, however, prevents it from quickly responding to the sudden change in driving direction initiated by the motor or clutch, and the energy generated by the shock at the initiation of such reversal is temporarily stored in the wire, causing it to vibrate. This vibration results in noticeable deterioration of the copied image being produced.
In an effort to absorb the initial vibration of the wire, it has been known to furnish a tensioner utilizing a spring or the like. This arrangement does not, however, provide a satisfactory countermeasure because vibration is produced not only at the initiation of motive action but additionally in constant speed motion due to the lesser tensile force of the tensioner as compared with the tensile strength of the wire.
Recently, a device has been utilized wherein two pulleys are provided on an oscillating member or bracket to which a friction member applies a braking force or motion resistance. Each of the pulleys is coupled under pressure to wires on both the tight and loose sides thereof; the impulse or shock of switching the tight side of the wire to the loose side--and vice versa--is to be absorbed by the frictional resistance produced during oscillation of the oscillating member (as, for example, disclosed in Japanese Laid Open Patent Publication No. 118,058/1978.) This arrangement, however, results in increased costs due to the requirement for two or more pulleys.
Moreover, if a wire is used as the driving means, some means for adjusting the tensile force of the wire is needed. Should such adjustment be attempted by adjusting the pressure forces coupling the two pulleys, the structure of the oscillating member becomes substantially more complex.